Sonographic appearance of fibrous nodules in patellar clunk syndrome: a case report.

Patellar clunk syndrome is a rare complication causing pain after total knee arthroplasty. However, it is important to differentiate it from the other causes of patellofemoral pain. In this article, we apply ultrasonography for the first time to demonstrate the suprapatellar fibrous tissue causing this condition. A 50-year-old woman with rheumatoid arthritis had left total knee arthroplasty. The patient had excellent results until 2 years after surgery, when she complained of anterior knee pain. On physical examination, a patellar "catch" and "pop" were noted from 45 degrees flexion to full extension. Roentgenograms showed a radiolucent line at the upper pole of the patella and no subluxation of the patella on skyline view. Sagittal and transverse gray-scale sonographic images revealed an echogenic area attached to the quadriceps tendon. Corresponding color Doppler sonographic images showed marked vascularity in this echogenic area. The patient was treated with removal of the suprapatellar tissue and a patellar component revision. Intraoperatively, the suprapatellar tissue, which measured 4 x 4 x 2 cm, entered the notch area of the femoral component on flexion and caused the patellar "pop" on knee extension. Pain relief was obtained immediately after the operation when smooth gliding motion of the patella was restored.